Issue #25: fix bootstrap.sh to work for RHEL7

- Remove pungi from list of host dependencies
in the Makefile (the host dependencies are
fulfilled in order to build pungi itself
so pungi should not be included in this list)
- Remove the host_packages directory (the
copies of rpms therein are no longer needed
now that the epel repo is being enabled)
- RHEL no longer has an optional channel, so
we remove the section which subscribes to the
optional repo
- Pipe out epel repo config with generic url
so that the latest epel-release is always
installed, then enable the epel repo
- Install packages which are not included in
the default RHEL build (now installable via
yum install because epel repo is enabled)